Micromirrors and off-diagonal hinge structures for micromirror arrays in projection displays

ABSTRACT

A spatial light modulator is disclosed, along with methods for making such a modulator, that comprises an array of micromirrors each having a hinge and a micromirror plate held via the hinge on a substrate, the micromirror plate being disposed in a plane separate from the hinge and having a diagonal extending across the micromirror plate, the micromirror plate being attached to the hinge such that the micromirror plate can rotate along a rotation axis that is parallel to, but off-set from the diagonal of the micromirror plate. Also disclosed is a projection system that comprises such a spatial light modulator, as well as a light source, condensing optics, wherein light from the light source is focused onto the array of micromirrors, projection optics for projecting light selectively reflected from the array of micromirrors onto a target, and a controller for selectively actuating the micromirrors in the array.

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 10/343,307 to Huibers filed Jan. 29, 2003, which isa US national phase application of PCT/US01/24332 filed Aug. 3, 2001,the subject matter of which is incorporated herein by reference

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention is related generally to spatial lightmodulators, and, more particularly, to fabrications of spatial lightmodulators with hinge structures.

BACKGROUND OF THE INVENTION

[0003] Spatial light modulators (SLMs) are transducers that modulate anincident beam of light in a spatial pattern in response to an optical orelectrical input. The incident light beam may be modulated in phase,intensity, polarization, or direction. This modulation may beaccomplished through the use of a variety of materials exhibitingmagneto-optic, electro-optic, or elastic properties. SLMs have manyapplications, including optical information processing, display systems,and electrostatic printing.

[0004] An early SLM designed for use in a projection display system isdescribed by Nathanson, U.S. Pat. No. 3,746,911. The individual pixelsof the SLM are addressed via a scanning electron beam as in aconventional direct-view cathode ray tube (CRT). Instead of exciting aphosphor, the electron beam charges deflectable reflective elementsarrayed on a quartz faceplate. Elements that are charged bent towardsthe faceplate due to electrostatic forces. Bent and unbent elementsreflect parallel incident light beams in different directions. Lightreflected from unbent elements is blocked with a set of Schlieren stops,while light from bent elements is allowed to pass through projectionoptics and form an image on a screen. Another electron-beam-addressedSLM is the Eidophor, described in E. Baumann, “The Fischer large-screenprojection system (Eidophor)” 20 J.SMPTE 351 (1953). In that system, theactive optical element is an oil film, which is periodically dimpled bythe electron beam so as to diffract incident light. A disadvantage ofthe Eidophor system is that the oil film is polymerized by constantelectron bombardment and oil vapors result in a short cathode lifetime.A disadvantage of both of these systems is their use of bulky andexpensive vacuum tubes.

[0005] A SLM in which movable elements are addressed via electricalcircuitry on a silicon substrate is described in K. Peterson,“Micromechanical Light Modulator Array Fabricated on Silicon” 31 Appl.Phys. Let. 521 (1977). This SLM contains a 16 by 1 array of cantilevermirrors above a silicon substrate. The mirrors are made of silicondioxide and have a reflective metal coating. The space below the mirrorsis created by etching away silicon via a KOH etch. The mirrors aredeflected by electrostatic attraction: a voltage bias is applied betweenthe reflective elements and the substrate and generates an electrostaticforce. A similar SLM incorporating a two-dimensional array is describedby Hartstein and Peterson, U.S. Pat. No. 4,229,732. Although theswitching voltage of this SLM is lowered by connecting the deflectablemirror elements at only one corner, the device has low light efficiencydue to the small fractional active area. In addition, diffraction fromthe addressing circuitry lowers the contrast ratio (modulation depth) ofthe display.

[0006] Another SLM design is the Grating Light Value (GLV) described byBloom, et. al., U.S. Pat. No. 5,311,360. The GLV's deflectablemechanical elements are reflective flat beams or ribbons. Light reflectsfrom both the ribbons and the substrate. If the distance between surfaceof the reflective ribbons and the reflective substrate is one-half of awavelength, light reflected from the two surfaces adds constructivelyand the device acts like a mirror. If this distance is one-quarter of awavelength, light directly reflected from the two surfaces willinterfere destructively and the device will act as a diffractiongrating, sending light into diffracted orders. Instead of using activesemiconductor circuitry at each pixel location, the approach in the '360patent relies on an inherent electromechanical bistability to implementa passive addressing scheme. The bistability exists because themechanical force required for deflection is roughly linear, whereas theelectrostatic force obeys an inverse square law. As a voltage bias isapplied, the ribbons deflect. When the ribbons are deflected past acertain point, the restoring mechanical force can no longer balance theelectrostatic force and the ribbons snap to the substrate. The voltagemust be lowered substantially below the snapping voltage in order forthe ribbons to return to their undeflected position. Ceramic films ofhigh mechanical quality, such as LPCVD (low pressure chemical vapordeposition) silicon nitride, can be used to form the ribbons. However,there are several difficulties with the GLV. A problem is that a passiveaddressing scheme might not be able to provide high frame rates (therate at which the entire SLM field is updated). In addition, with apassive addressing scheme, the ribbons deflect slightly even when off.This reduces the achievable contrast ratio. Also, even though the deviceis substantially planar, light is scattered, as in the DMD, from areasbetween the pixels, further reducing the contrast ratio.

[0007] Another diffraction-based SLM is the Microdisplay, described inP. Alvelda, “High-Efficiency Color Microdisplays” 307 SID 95 Digest.That SLM uses a liquid crystal layer on top of electrodes arrayed in agrating pattern. Pixels can be turned on and off by applying appropriatevoltages to alternating electrodes. The device is actively addressed andpotentially has a better contrast ratio than the GLV. However, thedevice, being based on the birefringence of liquid crystals, requirespolarized light, reducing its optical efficiency. Furthermore, theresponse time of liquid crystals is slow. Thus, to achieve color, threedevices—one dedicated for each of the primary colors—must be used inparallel. This arrangement leads to expensive optical systems.

[0008] A silicon-based micro-mechanical SLM with a large fractionaloptically active area is the Digital Mirror Device (DMD), developed byTexas Instruments and described by Hornbeck, U.S. Pat. No. 5,216,537 andother references.

[0009] Therefore, what is needed is a spatial light modulator that has ahigh resolution, a high fill factor and a high contrast ratio. What isfurther needed is a spatial light modulator that does not requirepolarized light, hence is optically efficient and mechanically robust.

SUMMARY OF THE INVENTION

[0010] In one embodiment of the invention, a reflective micromirror isdisclosed. The micromirror comprises: a hinge; and a micromirror platehaving a diagonal, attached to the hinge such that the micromirror platecan pivot along a rotation axis that is parallel to, but off-set fromthe diagonal of the micromirror plate.

[0011] In another embodiment of the invention, a method for making amicromirror device is provided. The method comprises: providing asubstrate; depositing a first sacrificial layer; forming a micromirrorplate having a diagonal; depositing a second sacrificial layer; forminga hinge-structure on the substrate for holding the micromirror plateabove the substrate such that the micromirror plate can pivot along arotation axis that is parallel to, but off-set from the diagonal of themicromirror plate; and removing the first and second sacrificial layers.

[0012] In yet another embodiment of the invention, a reflectivemicromirror device is disclosed herein. The micromirror devicecomprises: a substrate; a micromirror plate having a diagonal, formedabove the substrate for reflecting an incident light; and ahinge-structure formed on the substrate for holding the micromirrorplate, wherein the hinge-structure further comprises: a hinge, that isattached to the micromirror plate such that the micromirror plate canpivot along the a rotation axis that is parallel to, but offset from thediagonal of the micromirror plate; and a hinge-support for holding thehinge, wherein the hinge support is curved at a natural resting state.

[0013] In yet another embodiment of the invention, a method for making amicromirror device is provided here. The method comprises: providing asubstrate; depositing a first sacrificial layer; forming a micromirrorplate on the first sacrificial layer for reflecting an incident light;depositing a second sacrificial layer on the micromirror plate; forminga hinge-structure on the second sacrificial layer for holding themicromirror plate such that the micromirror plate can pivot along anaxis that is parallel to but offset from an diagonal of the micromirrorplate, further comprising: depositing a first hinge-structure layerhaving an intrinsic positive tensile-strain; and depositing a secondhinge-structure layer on the first layer, wherein the second layer hasan intrinsic negative compression strain; removing the first and secondsacrificial layers such that the first and second hinge-structure layersare curved at their natural resting states, and the micromirror plateheld by the hinge-structure is not parallel to the substrate at itsnatural resting state.

[0014] In still yet another embodiment of the invention, a reflectivemicromirror device is disclosed herein. The device comprises: asubstrate; a micromirror plate for reflecting an incident light; and ahinge-structure formed on the substrate for holding the micromirrorplate, wherein the —hinge structure further comprises: a hinge attachedto the micromirror plate such that the micromirror can pivot along anaxis that is parallel to, but offset from a diagonal of the micromirrorplate; and one or more mirror stops that stop a rotation of themicromirror plate along the axis.

[0015] In yet another embodiment of the invention, a micromirror arrayis disclosed herein. The device comprises: a substrate; and a pluralityof micromirrors formed on the substrate, wherein each micromirrorfurther comprises: a micromirror plate for reflecting an incident light;and a hinge-structure formed on the substrate for holding themicromirror plate, wherein the hinge-structure further comprises: ahinge attached to the micromirror plate such that the micromirror canpivot along an axis that is parallel to, but offset from a diagonal ofthe micromirror plate; and one or more mirror stops that stop a rotationof the micromirror plate along the axis.

[0016] In yet another embodiment of the invention, a projector isdisclosed herein. The projector comprises: a light source for providingan incident light; and a micromirror array, further comprising: asubstrate; and a plurality of micromirrors formed on the substrate,wherein each micromirror further comprises: a micromirror plate forreflecting an incident light; and a hinge-structure formed on thesubstrate for holding the micromirror plate, wherein the hinge-structurefurther comprises: a hinge attached to the micromirror plate such thatthe micromirror can pivot along an axis that is parallel to, but offsetfrom a diagonal of the micromirror plate; and one or more mirror stopsthat stop a rotation of the micromirror plate along the axis.

[0017] In still yet another embodiment of the invention, a reflectivemicromirror device is disclosed herein. The device comprises: asubstrate; a hinge held above the substrate by two or more posts formedon the substrate; and a micromirror plate attached to the hinge at apoint not along a straight line between the two posts.

[0018] In yet another embodiment of the invention, a reflectivemicromirror device is disclosed herein. The device comprises: asubstrate; a hinge-structure formed on the substrate, furthercomprising: two or more posts formed on the substrate; a hinge-supportheld by the two posts above the substrate, the hinge support beingcurved at a natural resting state; and a hinge held by the hinge-supportabove the substrate; and a micromirror plate attached to the hinge at apoint that is not along a straight line between the two posts forreflecting an incident light, the micromirror plate having a defineddiagonal.

[0019] In another embodiment of the invention, a reflective micromirrordevice is disclosed herein. The device comprises: a substrate; amicromirror plate for reflecting an incident light; and ahinge-structure formed on the substrate for holding the micromirrorplate, wherein the hinge structure further comprises: two or more postsformed on the substrate; a hinge held by the posts above the substrate,the hinge being attached to the micromirror plate at a point that is notalong a straight line between the two posts; and one or more mirrorstops that stop a rotation of the micromirror plate along the axis.

[0020] In yet another embodiment of the invention, a micromirror arrayis disclosed herein. The micromirror array comprises: a substrate; and aplurality of micromirrors formed on the substrate, wherein eachmicromirror further comprises: a micromirror plate for reflecting anincident light; and a hinge-structure formed on the substrate forholding the micromirror plate, wherein the hinge-structure furthercomprises: two or more posts formed on the substrate; a hinge held bythe posts above the substrate, the hinge being attached to themicromirror plate at a point not along a straight line between the twoposts; and one or more mirror stops that stop a rotation of themicromirror plate along the axis.

[0021] In still yet another embodiment of the invention, a projector isdisclosed herein. The projector comprises: a light source for providingan incident light; and a micromirror array, further comprising: asubstrate; and a plurality of micromirrors formed on the substrate,wherein each micromirror further comprises: a micromirror plate forreflecting an incident light; and a hinge-structure formed on thesubstrate for holding the micromirror plate, wherein the hinge-structurefurther comprises: two or more posts formed on the substrate; a hingeheld by the posts above the substrate, the hinge being attached to themicromirror plate at a point that is not along a straight line betweenthe two posts; and one or more mirror stops that stop a rotation of themicromirror plate along the axis.

BRIEF DESCRIPTION OF DRAWINGS

[0022] While the appended claims set forth the features of the presentinvention with particularity, the invention, together with its objectsand advantages, may be best understood from the following detaileddescription taken in conjunction with the accompanying drawings ofwhich:

[0023]FIG. 1 diagrammatically illustrates an exemplary display systememploying a spatial light modulator;

[0024]FIG. 2 is a top-view of the spatial light modulator used in thedisplay system of FIG. 1;

[0025]FIG. 3A is a back-view of a set of micromirrors according to anembodiment of the invention;

[0026]FIG. 3B illustrates a hinge-structure of the micromirrors of FIG.3A;

[0027]FIG. 3C is a back-view of a set of micromirrors according toanother embodiment of the invention;

[0028]FIG. 3D shows a hinge-structure of the micromirrors of FIG. 3C;

[0029]FIG. 3E illustrates therein a hinge-structure according to yetanother embodiment of the invention;

[0030]FIG. 3F illustrates therein a hinge-structure according to yetanother embodiment of the invention;

[0031]FIG. 3G illustrates therein a hinge-structure according to anotherembodiment of the invention;

[0032]FIG. 4A is a cross-sectional view of the micromirror device in an“OFF” state;

[0033]FIG. 4B is a cross-sectional view of the micromirror device inanother “OFF” state;

[0034]FIG. 4C is a cross-sectional view of the micromirror device in an“ON” state;

[0035]FIG. 4D is a cross-sectional view of the micromirror device in yetanother “OFF” state, wherein the hinge-structure has two sets of mirrorstops;

[0036]FIG. 4E is a cross-sectional view of another embodiment of themicromirror device with the mirror in the “ON” state;

[0037]FIG. 5A is a cross-sectional view of a micromirror device having ahinge-support that curves at a natural resting state;

[0038]FIG. 5B is a cross-sectional view of an exemplary hinge-supportbefore releasing according to an embodiment of the invention;

[0039]FIG. 5C is a cross-sectional view of the hinge-support of FIG. 5Bafter releasing;

[0040]FIG. 6A to FIG. 6H are cross-sectional views of structuresillustrating a method for forming a micromirror device according anembodiment of the invention;

[0041]FIG. 7A to FIG. 7B are cross-sectional views of structuresillustrating another method for forming a micromirror device accordingto another embodiment of the invention; and

[0042]FIG. 8 presents a cross-sectional view of a micromirror deviceafter releasing by removing the sacrificial layers.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] Processes for micro-fabricating a MEMS device such as a movablemicromirror and mirror array are disclosed in U.S. Pat. Nos. 5,835,256and 6,046,840 both to Huibers, the subject matter of each beingincorporated herein by reference. A similar process for forming MEMSmovable elements (e.g. mirrors) on a wafer substrate (e.g. a lighttransmissive substrate or a substrate comprising CMOS or othercircuitry) is illustrated in the present application. By “lighttransmissive”, it is meant that the material will be transmissive tolight at least in operation of the device (The material couldtemporarily have a light blocking layer on it to improve the ability tohandle the substrate during manufacture, or a partial light blockinglayer for decreasing light scatter during use. Regardless, a portion ofthe substrate, for visible light applications, is preferablytransmissive to visible light during use so that light can pass into thedevice, be reflected by the mirrors, and pass back out of the device. Ofcourse, not all embodiments will use a light transmissive substrate). By“wafer” it is meant any substrate on which multiple micromirrors ormicrostructure arrays are to be formed and which allows for beingdivided into dies, each die having one or more micromirrors thereon.Though not in every situation, often each die is one device or productto be packaged and sold separately. Forming multiple “products” or dieson a larger substrate or wafer allows for lower and faster manufacturingcosts as compared to forming each die separately. Of course the waferscan be any size or shape, though it is preferred that the wafers be theconventional round or substantially round wafers (e.g. 4″, 6″ or 12″ indiameter) so as to allow for manufacture in a standard foundry.

[0044] U.S. patent application Ser. No. 09/910,537 filed Jul. 20, 2001,and 60/300,533 filed Jun. 22, 2001 both to Reid contain examples ofmaterials that may be used for the various components of the currentinvention. These applications are incorporated herein by reference.

[0045] The present invention provides a spatial light modulator that hasa higher resolution, an increased fill factor, and an increased contrastratio in displaying an image. The spatial light modulator may beoperated in the absence of polarized light. Moreover, the spatial lightmodulator has improved electromechanical performance and robustness withrespect to manufacturing.

[0046] The spatial light modulator of the present invention has avariety of applications (e.g. maskless lithography, atomic spectroscopy,maskless fabrication of micromirror arrays, signal processing,microscopy etc), one of which is in display systems. A typical displaysystem employing a spatial light modulator is illustrated in FIG. 1. Inits very basic configuration, the display system comprises light source120, optical devices (e.g. light pipe 150, collection optics 160 andprojection optics 190), display target 210 and spatial light modulator200. Light source 120 (e.g. an arc lamp) directs light through the lightintegrator/pipe 150 and collection optics 160 and onto spatial lightmodulator 200. The micromirrors of the spatial light modulator 200 areselectively actuated by a controller (e.g. as disclosed in U.S. Pat. No.6,388,661 issued May 14, 2002 incorporated herein by reference) so as toreflect—when in their “ON” position—the incident light into projectionoptics 190, resulting in an image on display target 210 (screen, aviewer's eyes, a photosensitive material, etc.). Of course, more complexoptical systems are often used the display system of FIG. 1 being asimplification of a typical projection display optical system.

[0047] The spatial light modulator, in general, comprises an array ofthousands or millions of micromirrors. FIG. 2 illustrates a portion ofan exemplary micromirror array. Referring to FIG. 2, a top-view of aportion of an exemplary spatial light modulator 200 looking throughglass is illustrated therein. As shown, the spatial light modulatorcomprises micromirror array 201 that is formed on a substrate 202, suchas glass that is visible light transmissive. Alternatively, substrate202 is a typical semiconductor wafer that has formed thereon an array ofelectrodes and circuitry (not shown in FIG. 2) for electrostaticallycontrolling motions of the micromirrors. Micromirror array 201 comprisesa plurality of micromirror devices, such as micromirror device 215. Andeach micromirror device further comprises a reflective micromirrorplate, such as micromirror plate 210 for reflecting the incident light.In operation, each individual micromirror can be deflected as desiredunder the control of one or more electrodes and circuitry, thereby thespatial modulation of the incident light traveling through substrate 202(in this case, the substrate is a glass) and incident on the surfaces ofthe micromirrors can be achieved. To facilitate the micromirror platerotating above the substrate (or below, depending upon the point ofview) for reflecting the incident light, a hinge-structure is necessaryto hold the micromirror plate above the substrate and provide a rotationaxis for the micromirror plate.

[0048] Referring to FIG. 3A, a back-view of the micromirror array (e.g.201) shown in FIG. 2 is illustrated therein. Each micromirror plate(e.g. micromirror 210) is attached to a hinge-structure (e.g. hingestructure 230) such that the micromirror plate can pivot along the hingestructure above the substrate (e.g. substrate 202 in FIG. 2). In orderto improve the quality of displayed images, the hinge structure ispreferably formed under the micromirror plates as shown. Specifically,the hinge structure and the surface for reflecting the incident lightare on the opposite sides of the micromirror plate.

[0049] According to an embodiment of the invention, the micromirrorplate is attached to the hinge structure such that the micromirror platecan pivot along an axis that is parallel to but offset from a diagonalof the micromirror plate. For example, micromirror plate 210 has a welldefined geometrical diagonal 211. However, the rotation axis of themicromirror plate is along direction 213 that is parallel to but offsetfrom diagonal 211. Such a rotation axis can be achieved by attaching thehinge structure to the mirror plate at a point not along the mirrorplate diagonal 211. The point of attachment can be at least 0.5 um, atleast 1 um, or at least 2 um away from the diagonal 211. In oneembodiment, the point of attachment is from {fraction (1/40)} to ⅓ thelength of the diagonal away from diagonal 211, or from {fraction (1/20)}to ¼ if desired—although any desired distance away from the diagonal ispossible if so desired in the present invention. In the presentinvention, the micromirror preferably has a substantially four-sidedshape. Whether the micromirror is a rectangle, square, rhombus ortrapezoid, even if the corners are rounded or “clipped” or if anaperture or protrusion is located on one or more of the sides of themicromirror, it is still possible to conceptually connect the four majorsides of the micromirror shape and take a diagonal across the middle ofthe micromirror. In this way, a center diagonal can be defined even ifthe micromirror plate is substantially but not perfectly a rhombus,trapezoid, rectangle, square, etc. However, the rotation axis of themicromirror plate is not along the center diagonal but is alongdirection 213 that is parallel to but offset from diagonal 211 in FIG.3A. By “parallel to but offset from the diagonal”, it is meant that theaxis of rotation can be exactly parallel to or substantially parallel to(19 degrees) the diagonal of the micromirror. This type of designbenefits the performance of the micromirror device in a number of ways.One advantage of this asymmetric offset arrangement is that themicromirror plate can rotate at a larger angle than the rotation anglethat can be achieved in a symmetrical arrangement (with a mirrorplate—substrate gap being the same). The length of the diagonal of themirror plate is preferably 25 microns or less.

[0050] In order to hold the micromirror plate and meanwhile, provide arotation axis for the micromirror plate for rotating above thesubstrate, each hinge structure, such as hinge structure 230, furthercomprises hinge-support 250 and hinge 240, as shown in FIG. 3B. Hinge240 is attached to the micromirror plate via contact 257. Hinge support250 further comprises two posts 251. By “hinge” is meant the layer orstack of layers that defines that portion of the device that flexes toallow movement of the device (described in detail below). To improve theperformance of the micromirror plate, further fine structures are alsoprovided thereon. Specifically, two mirror stops 255 are formed on anedge of hinge support 250 for stopping the rotation of the micromirrorplate when the micromirror plate achieves a certain angle. Thegeometrical arrangement, such as the length and the position of themirror stop from the hinge-plate, along with the distance between themicromirror plate and the hinge determines the maximum rotation anglethat the micromirror can achieve before contact. By properly setting themirror stops for all micromirror plates in the micromirror array, amaximum rotation angle for all micromirrors can be uniformly defined.This uniformly defined rotation angle can then be defined as an “ON”state for all micromirrors in operation. In this case, all micromirrorsin the spatial light modulator rotate to the uniformly defined angle inthe “ON” state in an operation. The incident light can thus be uniformlyreflected towards one desired direction for display. Obviously, thissignificantly improves the quality of the displayed image. Thoughpreferred, the number of the mirror stops can be of any desired number(one or more) or need not be provided at all. And each mirror stop canbe of any desired shape, though preferably one that minimizes the amountof contact between the mirror stop and the micromirror plate.

[0051] In the embodiment of the invention, the two posts are formed onthe substrate. Hinge-support 250 is supported by the two posts above thesubstrate. The hinge (e.g. hinge 240) is affixed to the hinge-supportand attached to the micromirror plate via the contact (e.g. contact255). In this configuration, the micromirror plate can pivot along thehinge above the substrate.

[0052] The hinge structure can take other suitable forms as desired.FIG. 3C illustrates another hinge structure design according to anotherembodiment of the invention. Similar to that of FIG. 3A, hinge structure260 is formed on the substrate for supporting micromirror plate 210 andprovides a rotation axis 214 for the micromirror plate. Rotation axis214 is parallel to, but offset from a diagonal of micromirror plate 212.Similar to hinge-support 250 in FIG. 3B, hinge-support 263 in FIG. 3Dalso has a plurality of mirror stops formed thereon for stopping therotation of the micromirror plate when the micromirror plate achieves acertain angle. The geometrical arrangement, such as the length and theposition on the hinge-support, along with the distance between themicromirror plate and the hinge determines the maximum rotation anglethat the micromirror can achieve before contact. Though preferred, thenumber of the mirror stops can be of any desired number. And each mirrorstop can be of any desired shape.

[0053] The hinge structure can also take other suitable forms. Forexample, hinge support 261 can be formed along the edges of one part ofthe micromirror plate such that the hinge-support passes through a postof adjacent micromirror device, as shown in FIG. 3E In this case, thehinge-support of all micromirror devices form a continuous hinge-supportframe for all micromirror plates. This allows 2-dimensional electricalconnection of the micromirrors in the array.

[0054] Alternatively, the posts of each hinge structure are not requiredto be formed along the diagonal of the micromirror plate. Referring toFIG. 3F, two posts 251 of the hinge structure are formed along the edgesof the micromirror plate instead of at the corners of the micromirrorplate. In addition, the hinge is not required to be placed such that thehinge and the two arms intersected with the hinge form an isoscelestriangle, as shown in the figure. Instead, as shown in FIG. 3G, thehinge may be placed such that it is substantially parallel but forms asmall angle (±19 degrees) with the hinge position in FIG. 3F.

[0055] In operation, the micromirror plate rotates along the hinge thatis parallel to but offset from a diagonal of the micromirror plate.Based on rotation angles, “ON” and “OFF” states are defined. At the “ON”state, the micromirror plate is rotated to a predefined angle such thatthe incident light can be reflected into a direction for view, forexample, into a set of pre-arranged optic devices for directing lighttowards a target. In the “OFF” state, the micromirror plate stays flator at another angle such that the incident light will be reflected awayfrom the display target. FIG. 4A through FIG. 4D illustratecross-sectional views of a micromirror device in operation.

[0056] Referring to FIG. 4A, an “OFF” state is define as micromirrorplate 210 at its natural resting state that is parallel to glasssubstrate 280. Hinge support 263 is formed on the substrate forsupporting the micromirror plate. The hinge (e.g. hinge 240 in FIG. 3B)is affixed to hinge-support 263 and attached to micromirror plate 210via shallow via contact 241 (hereafter “contact”) for providing arotational axis for the micromirror plate. In this “OFF” state, theincident light travels through the glass substrate, shines on onesurface of the micromirror plate at a particular incident angle and isreflected away from the target by the micromirror plate. The rotation ofthe micromirror plate can be electrostatically controlled by electrode282 and a circuitry (not shown) that is connected to the electrode. Inan embodiment of the invention, the electrode and circuitry are formedin wafer 281, which can be a typical silicon wafer. In order toefficiently control the rotation of the micromirror plate, wafer 281 isplaced approximate to the micromirror plate such that electrostaticfields can be established between micromirrors and associatedelectrodes. Alternatively, more than one electrode can be used forcontrolling the rotation of the micromirror plate. Specifically,electrode 283 (and circuitry connected to the electrode, which is notshown) can be formed and placed underneath the other portion of themicromirror plate for controlling the micromirror plate in an “OFF”state, as shown in FIG. 4B. In another embodiment of the invention, theelectrodes, the circuitry and the micromirrors can be formed on the samesubstrate, such as substrate 280. In this case, substrate 280 can be astandard silicon wafer. And the incident light shines the oppositesurface of the micromirror plate. To improve the quality of thedisplayed image, especially the contrast ratio, it is desired that thereflected light in the “OFF” state be reflected as much as possible awayfrom the collection optics or target. To achieve this, another “OFF”state is defined as shown in FIG. 4B. Referring to FIG. 4B, micromirrorplate 210 is rotated at an angle in the “OFF” state. As an optionalfeature, the angle corresponding to this “OFF” state is defined suchthat one end of the micromirror plate touches and is stopped by thesubstrate when the micromirror plate is rotated to this angle. Thisdefinition ensures a uniform “OFF” state for all micromirror plates inthe micromirror array. Of course, other methods can also be employed indefining an “OFF” state angle. For example, by properly controlling theelectric field applied between the micromirror plate and theelectrode(s) and circuitry associated with the micromirror plate,desired angles “corresponding to the “OFF” state can be achieved. Inorder to direct the reflected light into the target for displaying, themicromirror plate needs to be rotated to a certain angle, which iscorresponds to an “ON” state. FIG. 4C illustrates a cross-sectional viewof the micromirror device in an exemplary “ON” state according to anembodiment of the invention. In this “ON” state, the rotation of themicromirror plate is stopped by mirror stops 270. By adjusting theconfiguration (e.g. length and the position on the hinge structure) ofthe mirror stops, the angle corresponding to the “ON” state can thus beadjusted, as long as the other end of the micromirror plate is free tomove. The presence of the mirror stops benefit a uniform “ON” state forall micromirror plates in the spatial light modulator, thus, the qualityof the displayed image is significantly improved. As an optional featureof the embodiment, the mirror stops can be designed and formed such thatthe other end of the micromirror plate touches and is stopped by thesubstrate when the rotation of the micromirror plate touches and isstopped by the mirror stops, as shown in FIG. 4C. This dual-stoppingmechanism further guarantees a uniform rotation angle corresponding tothe “On” state for all micromirror plates. As a further optionalfeature, another set of mirror stops for the “OFF” state may also beprovided in addition to the set of mirror stops for the “ON” state, asshown in FIG. 4D.

[0057] Referring to FIG. 4D, a first set of mirror stops 270 is formedon the hinge structure for providing a uniform “ON” state for allmicromirror plates. And a second set of mirror stops 275 is furtherprovided for ensuring a uniform “OFF” state for all micromirror plates.The physical properties (e.g. length and position) of the second set ofmirror stops 275 determine the rotation position of the “OFF” state.Alternatively, the second set of mirror stops can be designed and formedsuch that the other end of the micromirror plate touches and is stoppedby the glass substrate when the micromirror plate touches and is stoppedby the second set of mirror stops.

[0058] In operation, the micromirror plate (e.g. 210 in FIG. 3C) pivotsalong the hinge and reflects incident light. This type of operationmechanism calls for certain requirements on the optical, mechanical andelectric properties of the micromirror plate, hinge structure andcontact 255. In particular, the micromirror plate is desired to comprisea material having high reflectivity to the light of interest, forexample, a material of early transition metal, metal or metal alloy. Inaddition, it is desired that the material of the micromirror plate alsoexhibits suitable mechanical properties (e.g. low creep rate and highelastic modulus etc.) for enhancing the mechanical property of themicromirror plate. Furthermore, it is desired that the material of themicromirror plate is electrically conductive such that an electricvoltage can be applied thereto.

[0059] The hinge-support (e.g. 260 in FIG. 3C) provides an axis by whichthe micromirror plate (e.g. micromirror plate 210) can rotate. Becausethe hinge-support may scatter incident light and the scattered light canbe mingled with the reflected light, thereby, the contrast ration can bedegraded. In order to suppress this type of scattering, the hingestructure is preferably “bidden” beneath the micromirror plate. Forexample, the hinge structure is formed on a side of the micromirrorplate that is opposite to the side of the micromirror plate reflectingthe incident light. In accordance with the operation mechanism of themicromirror plate and the constructional design, it is desired that theposts comprise materials that are insusceptible to elastic deformation(e.g. fatigue, creep, dislocation motion) during the operation of thedevice. It is also preferred that such materials have large elasticmodulus and exhibits high stiffness. Opposite to that of the posts, thematerials of the hinge (e.g. hinge 240 in FIG. 3D) are expected to bemore compliant because the hinge deforms while the micromirror platepivots. Moreover, the hinge is desired to be electrically conductingsuch that the micromirror plate can be held at a particular voltagelevel.

[0060] In order to achieve the defined “OFF” states in FIG. 4B and FIG.4D, external forces (e.g. electrical fields) may required to rotate themicromirror plate. For example, an electrode 283 and circuitry may bedisposed underneath the portion of the micromirror plate being rotatedaway from the substrate. An electric field can then be applied betweenthe electrode and the portion of the micromirror plate for rotating themicromirror plate to the “OFF” state. This design, however, requiresextra electrodes and circuitry.

[0061] According to an aspect of the invention, a hinge-support with aportion that is curved away from the substrate at the natural restingstate is proposed, as shown in FIG. 5A. Referring to FIG. 5A,hinge-support portion 250 is curved away from the substrate at itsnatural resting state. And micromirror plate 210, which is attached tothe curved hinge-support, presents a finite angle with the substratewithout external force (e.g. external electrical field). By adjustingthe curvature of the hinge-support portion, a desired angle between themicromirror plate and the substrate can be achieved.

[0062] The curved hinge-support can be formed in many different ways. Anexemplary method will be discussed in the following with references toFIG. 5b and FIG. 5C. Referring to FIG. 5B, hinge-support 250 composestwo layers, layer 251 and layer 253. Layer 251 exhibits an outwardscompression strain at its deposition state (e.g. when layer 251 isdeposited on a sacrificial layer). In the preferred embodiment of theinvention, layer 251 is TiN_(x) with a preferred thickness of 80 Å.Though preferred, layer 251 can be of any suitable material as long asit exhibits an outwards compression strain. The thickness of layer 251can also be of any suitable range, such as a thickness between 10 Å to1500 Å. Layer 253 is deposited on layer 251 and exhibits an inwardstensile strain at its deposition state. In a preferred embodiment of theinvention, layer 253 is SiN_(x) with a preferred thickness of 400 Å.Though preferred, layer 253 can be of any suitable material as long asit exhibits an inwards tensile strain. The thickness of layer 253 canalso be of any suitable range, such as a thickness between 10 Å to 2000Å. PVD (physical vapor deposition or sputtering) tends to producecompressive films, especially for high melting temperature metals,whereas CVD (chemical vapor deposition) tends to produce tensile films.Therefore, in one embodiment layer 251 is a layer deposited by PVD andlayer 253 is deposited by CVD. In one specific example, layer 251 is areactively sputtered ceramic layer and layer 253 is a ceramic layerdeposited by chemical vapor deposition.

[0063] After releasing, (for example, by removing the sacrificial layer,on which layer 251 is deposited), layers 253 and 251 curve towards layer253, which exhibits inwards tensile strain. This curving of the twolayers is a spontaneous phenomenon and happens in the presence ofmaterial stresses. The curvature is determined upon the relativestrengths of the inwards tensile strain and outwards compression strain.Referring to FIG. 5C, a schematic diagram showing the curved two layersis presented therein. However, depending upon the location of the hingeconnection to the mirror plate, the order of the layers can be reversedin order to cause curvature of the hinge structure in the oppositedirection while rotating the mirror plate in the same direction for the“OFF” state.

[0064] There is a variety of ways to construct the micromirror devicedescribed above. Exemplary processes will be discussed in the followingwith references to FIG. 6A through FIG. 6H. It should be appreciated bythose ordinary skills in the art that the exemplary processes are fordemonstration purpose only and should not be interpreted as limitations.

[0065] Referring to FIG. 6A, substrate 280 is provided. Firstsacrificial layer 290 is deposited on the substrate followed by thedeposition of micromirror plate layer 300. The substrate can be a glass(e.g. 1737F, Eagle 2000), quartz, Pyrex™, sapphire. The substrate mayalso be a semiconductor substrate (e.g. silicon substrate) with one ormore actuation electrodes and/or control circuitry (e.g. CMOS type DRAM)formed thereon.

[0066] First sacrificial layer 290 is deposited on substrate 280. Firstsacrificial layer 290 may be any suitable material, such as amorphoussilicon, or could alternatively be a polymer or polyimide, or evenpolysilicon, silicon nitride, silicon dioxide, etc. depending upon thechoice of sacrificial materials, and the etchant selected. If the firstsacrificial layer is amorphous silicon, it can be deposited at 300-350°C. The thickness of the first sacrificial layer can be wide rangingdepending upon the micromirror size and desired title angle of themicro-micromirror, though a thickness of from 500 Å to 50,000 Å,preferably around 10,000 Å, is preferred. The first sacrificial layermay be deposited on the substrate using any suitable method, such asLPCVD or PECVD.

[0067] As an optional feature of the embodiment, anti-reflection layer285 maybe deposited on the surface of the substrate. The anti-reflectionlayer is deposited for reducing the reflection of the incident lightfrom the surface of the substrate. Alternatively, other opticalenhancing layers may be deposited on either surface of the glasssubstrate as desired.

[0068] After depositing the first sacrificial layer, a plurality ofstructure layers will be deposited and patterned as appropriate.According to the invention, a structural layer is a layer that will notbe removed after the removal of the sacrificial layers. The firststructural layer deposited on the first sacrificial layer is micromirrorplate layer 300 for forming a micromirror. Because the micromirror isdesignated for reflecting incident light in the spectrum of interest(e.g. visible light spectrum), it is preferred that the micromirrorplate layer comprises of one or more materials that exhibit highreflectivity (preferably 90% or higher) to the incident light. Accordingto the embodiment of the invention, micromirror plate layer 300 is amulti-layered structure as shown in FIG. 6B. Referring to FIG. 6B, hingeplate layer 300 comprises layers 307, 305, 303 and 301. Layers 307 and301 are protection layers for protecting the interior layers (e.g.layers 303 and 305). In the preferred embodiment of the invention,layers 307 and 301 are SIO_(x) with a preferred thickness of 400 Å. Ofcourse, other suitable materials may also be employed herein. Layer 305is a light reflecting layer that comprises one or more materialsexhibiting high light reflectivity. Examples of such materials are Al,Ti, AlSiCu or TiAI. In the preferred embodiment of the invention, layer305 is aluminum with a thickness of 2500 Å. This aluminum layer ispreferred to be deposited at 150° C. or other temperatures preferablyless than 400° C. Layer 303 is an enhancing layer that comprises ofmetal or metal alloy for enhancing the electric and mechanicalproperties of the micromirror plate. An example of such enhancing layeris titanium with a thickness of 80 Å. Of course, other suitablematerials having high reflectivity to the incident light of interest mayalso be adopted for the micromirror plate. In depositing the micromirrorplate layer, PVD is preferably used at 150° C. The thickness of themicromirror plate layer can be wide ranging depending upon the desiredmechanical (e.g. elastic module), the size of the micromirror, desiredtitled angle and electronic (e.g. conductivity) properties of themicromirror plate and the properties of the materials selected forforming the micromirror plate. According to the invention, a thicknessof from 500 Å to 50,000 Å, preferably around 2500 Å, is preferred.

[0069] According to another embodiment of the invention, the lightreflecting layer 305 is an electro-conducting layer that comprises amaterial having a resistivity less than 10,000 μΩ∘cm. Layers 301 and 307are insulators with resistivities greater than 10,000 μ106 ∘cm. Andlayer 303 is an electro-conducting layer with a resistivity also lessthan 10,000 μ106 ∘cm.

[0070] Though preferred, the multilayered structure as shown in FIG. 6Bcomprises four layers. It will be appreciated by those ordinary skillsin the art that the number of the multilayered structure should not beinterpreted as a limitation. Instead, any number of layers (including asingle layer) can be employed without departing from the spirit of thepresent invention.

[0071] Micromirror plate layer 300 is then patterned into a desiredshape, as shown in FIG. 6C. The micromirror can be of any shape asdesired. The patterning of the micromirror can be achieved usingstandard photoresist patterning followed by etching using, for exampleCF4, C12, or other suitable etchant depending upon the specific materialof the micromirror plate layer.

[0072] After the formation of the micromirror plate, further structurallayers are deposited and patterned. Specifically, a plurality of layersof the hinge structure will be deposited and patterned for forming thehinge structure. Referring to FIG. 6D, before depositing furtherstructural layers, second sacrificial layer 310 is deposited on top ofthe micromirror plate 300 and first sacrificial layer 290. Secondsacrificial layer 310 may comprise amorphous silicon, or couldalternatively comprise one or more of the various materials mentionedabove in reference to first sacrificial layer 290. First and secondsacrificial layers need not be the same, though are the same in thepreferred embodiment so that, in the future, the etching process forremoving these sacrificial layers can be simplified. Similar to thefirst sacrificial layer, second sacrificial layer 310 may be depositedusing any suitable method, such as LPCVD or PECVD. If the secondsacrificial layer comprises amorphous silicon, the layer can bedeposited at 350° C. The thickness of the second sacrificial layer canbe on the order of 9000 Å, but may be adjusted to any reasonablethickness, such as between 2000 Å and 20,000 Å depending upon thedesired distance (in the direction perpendicular to the micromirrorplate and the substrate) between the micromirror plate and the hinge. Itis preferred that the hinge and mirror plate be separated by a gap afterrelease of at least 0.5 um (this can be at least 1 um or even 2 um ormore if desired). Second sacrificial layer 310 may also fill in thetrenches left from the patterning of the micromirror plate.

[0073] In the preferred embodiment of the invention, the micromirrorplate layer comprises an aluminum layer (e.g. layer 305 in FIG. 6B), andthe second sacrificial layer is silicon. This design, however, can causedefects in the hinge-structure due to the diffusion of the aluminum andsilicon at the edges of the micromirror plate, wherein the aluminum isexposed to the silicon. To solve this problem, a protection layer (notshown) maybe deposited on the patterned micromirror plate beforedepositing the second sacrificial silicon layer such that the aluminumlayer can be isolated from the silicon sacrificial layer. Then theprotection layer is patterned according to the shape of the micromirrorplate. After the patterning, segments of the protection layer (e.g.segment 211 in FIG. 6C) cover the edges of the micromirror plate forisolating the aluminum and the silicon sacrificial layer.

[0074] The deposited second sacrificial layer is patterned afterwardsfor forming two deep-via areas 320 and shallow via area 330 usingstandard lithography technique followed by etching, as shown in FIG. 6E.The etching step may be performed using Cl₂, BCI₃, or other suitableetchant depending upon the specific material(s) of the secondsacrificial layer. The distance across the two deep-via areas 320depends upon the length of the defined diagonal of the micromirrorplate. In an embodiment of the invention, the distance across the twodeep-via areas after the patterning is preferably around 10 μm, but canbe any suitable distance as desired. In order to form shallow-via area330, an etching step using CF₄ or other suitable etchant may beexecuted. The shallow-via area, which can be of any suitable size, ispreferably on the order of 2.2 μm on a side.

[0075] Referring to FIG. 6F, hinge-support layers 340 and 350 aredeposited on the patterned second sacrificial layer 310. Because thehinge-support layers (layers 340 and 350) are designated for holding thehinge (e.g. 240 in FIG. 3D) and the micromirror plate (e.g. 210 in FIG.3C) attached therewith such that the micromirror plate can pivot alongthe hinge, it is desired that the hinge support layers comprise ofmaterials having at least large elastic modulus. According to anembodiment of the invention, layer 340 comprises a 400 Å thickness ofTiN_(x) (although it may comprise TiN_(x), and may have a thicknessbetween 100 Å and 2000 Å) layer deposited by PVD, and a 3500 Å thicknessof SiN_(x), (although the thickness of the SiN_(x) layer may be between2000 Å and 10,000 Å) layer 350 deposited by PECVD. Of course, othersuitable materials and methods of deposition may be used (e.g. methods,such as LPCVD or sputtering). The TiN_(x) layer is not necessary for theinvention, but provides a conductive contact surface between themicromirror and the hinge in order to, at least, reduce charge-inducedstiction. According to the embodiment of the invention, layers 340 and350 are deposited such that an inwards compression strain and outwardstensile strain are inherently presented for forming a curvedhinge-support (e.g. 250 in FIG. 5A), as shown in FIG. 5C. Alternatively,the TiN_(x) and SiN_(x) layers can also be deposited such that theintrinsic stress is as low as possible, preferably lower than 250 MPafor forming a flat hinge-support. In either case, the SiN_(x) layer canbe deposited at 400° C.

[0076] After the deposition, layers 340 and 350 are patterned into adesired configuration (e.g. hinge support 275 in FIG. 3D), as shown inFIG. 6G. Posts 260 can take any desired forms, one of which is shown inFIG. 3D. Alternatively, each of the two posts may be formed as adiamond, such as posts 251 in FIG. 3F. The mirror stops, such as themirror stops (e.g. mirror stops 270 in FIG. 3D) corresponding to the“ON” state and/or mirror stops (not shown) corresponding to the “OFF”state can also be configured. An etching step using one or more properetchants is then performed afterwards. In particular, the layers can beetched with a chlorine chemistry or a fluorine chemistry where theetchant is a perfluorocarbon or hydrofluorocarbon (or SF₆) that isenergized so as to selectively etch the hinge support layers bothchemically and physically (e.g. a plasma/RIE etch with CF₄, CHF₃, C₃F₈,CH₂F₂, C₂F₆, SF₆, etc. or more likely combinations of the above or withadditional gases, such as CF₄/H₂, SF₆/CI₂, or gases using more than oneetching species such as CF₂CI₂, all possibly with one or more optionalinert diluents). Different etchants may, of course, be employed foretching each hinge support layer (e.g. chlorine chemistry for a metallayer, hydrocarbon or fluorocarbon (or SF₆) plasma for silicon orsilicon compound layers, etc.). Alternatively, the etching step can beperformed after deposition of each hinge support layer. For example,layer 340 can be etched and patterned after the deposition of layer 340and before the deposition of layer 350.

[0077] After etching layers 340 and 350, two posts 260 and a contactarea 330 are formed. The bottom segment of contact area 330 is removedby etching and the part of the micromirror plate underneath the contactarea is thus exposed. The exposed part of micromirror 210 will be usedto form an electric-contact with external electric source. The sidewalls(e.g. 335) of contact area 330 are left with residues of layers 340 and350 after etching. The residue 335 has a slope measured by angle 0approximately 75 degrees, but may vary between 0 and 89 degrees. Theresidue on the sidewalls helps to enhance the mechanical and electricalproperties of the hinge that will be formed afterwards. Each of the twoposts 260 on either side of the mirror can form a continuous elementwith the posts corresponding to the adjacent micromirror in an array asshown in FIG. 2.

[0078] After the completion of patterning and etching of layers 340 and350, hinge layer 360 is deposited and then patterned as shown in FIG.6H. Because the hinge provides a rotation axis for the micromirrorplate, it is natural to expect that the hinge layer comprises a materialthat is at least susceptible to plastic deformation (e.g. fatigue,creep, and dislocation motion). Furthermore, when the hinge layer isalso used as electric contact for the micromirror plate, it is desiredthat the material of the hinge layer is electrically conductive.Examples of suitable materials for the hinge layer are silicon nitride,silicon oxide, silicon carbide, polysilicon, Al, Ir, titanium, titaniumnitride, titanium oxide(s), titanium carbide, CoSiN_(x), TiSiN_(x),TaSiN_(x), or other ternary and higher compounds. When titanium isselected for the hinge layer, it can be deposited at 100° C.Alternatively, the hinge layer may comprise of multi-layers, such as 100Å TiN_(x) and 400 Å SiN_(x).

[0079] After deposition, the hinge layer is then patterned as desiredusing etching. Similar to the hinge layers (layers 340 and 350), thehinge layer can be etched with a chlorine chemistry or a fluorinechemistry where the etchant is a perfluorocarbon or hydrofluorocarbon(or SF₆) that is energized so as to selectively etch the hinge layersboth chemically and physically (e.g. a plasma/RIE etch with CF₄, CHF₃,C₃F₈, CH₂F₂, C₂F₆, SF₆, etc. or more likely combinations of the above orwith additional gases, such as CF₄/H₂, SF₆/CI₂, or gases using more thanone etching species such as CF₂CI₂, all possibly with one or moreoptional inert diluents). Different etchants may, of course, be employedfor etching each hinge layer (e.g. chlorine chemistry for a metal layer,hydrocarbon or fluorocarbon (or SF₆) plasma for silicon or siliconcompound layers, etc.).

[0080] In order to release the micromirror plate for pivoting along thehinge, the sacrificial layers (e.g. layers 290 and 310) are removed byetching as discussed below. A cross-sectional view of the releasedmicromirror device is presented in FIG. 8.

[0081] In the above described exemplary fabrication process, theprocesses for forming the hinge support (e.g. processes described inFIG. 6A to FIG. 6G) and the process for forming the hinge (e.g. processdescribed in FIG. 6H) are performed consecutively. In particular, thepatterning and etching of the hinge support is followed by thedeposition, patterning and etching of the hinge. The hinge and the hingesupport can be formed simultaneously according to another embodiment ofthe invention, which will be described in the following with referencesto FIG. 7A and FIG. 7B.

[0082] Referring to FIG. 7A, the deposited hinge layers 340 and 350 forthe hinge support (e.g. 275 in FIG. 3D) are first patterned and etchedaccording to the desired configuration of the hinge. After etching,window 370 corresponding to the future location of the hinge (e.g. hinge240 in FIG. 3D) is thus formed thereby. Window 370 is disposed parallelto but offset from a diagonal of the micromirror plate. The window isetched down to the top surface of the second sacrificial layer (e.g. 310in FIG. 6D) and/or micromirror plate such that the bottom of the windowexposes a part of the micromirror plate.

[0083] Following the completion of the patterning, hinge layer 360 isdeposited on the patterned hinge support layer (e.g. 350) and fillswindow 370. After deposition, layer 340, 350 and 360 are then patternedand etched simultaneously. In a preferred embodiment of the invention,layers 340 and 350 are patterned and etched simultaneously using thesame method that is described in FIG. 6G. After patterning and etchingof layers 340 and 350, the sacrificial layers are removed by etching forreleasing the micromirror device.

[0084] The release etching utilizes an etchant gas capable ofspontaneous chemical etching of the sacrificial material, preferablyisotropic etching that chemically (and not physically) removes thesacrificial material. Such chemical etching and apparatus for performingsuch chemical etching are disclosed in U.S. patent application Ser. No.09/427,841 to Patel et al. filed Oct. 26, 1999, and in U.S. patentapplication Ser. No. 09/649,569 to Patel at al. filed Aug. 28, 2000, thesubject matter of each being incorporated herein by reference. Preferredetchants for the release etch are gas phase fluoride etchants that,except for the optional application of temperature, are not energized.Examples include HF gas, noble gas halides such as xenon difluoride, andinterhalogens such as IF₅, BrCl₃, BrF₃, IF₇ and CIF₃. The release etchmay comprise additional gas components such as N₂ or an inert gas (Ar,Xe, He, etc.). In this way, the remaining sacrificial material isremoved and the micromechanical structure is released. In one aspect ofsuch an embodiment, XeF₂ is provided in an etching chamber with diluents(e.g. N₂ and He). The concentration of XeF₂ is preferably 8 Torr,although the concentration can be varied from 1 Torr to 30 Torr orhigher. This non-plasma etch is employed for preferably 900 seconds,although the time can vary from 60 to 5000 seconds, depending ontemperature, etchant concentration, pressure, quantity of sacrificialmaterial to be removed, or other factors. The etch rate may be heldconstant at 18 Å/s/Torr, although the etch rate may vary from 1 Å/s/Torrto 100 Å/s/Torr. Each step of the release process can be performed atroom temperature.

[0085] In addition to the above etchants and etching methods mentionedfor use in either the final release or in an intermediate etching step,there are others that may also be used by themselves or in combination.Some of these include wet etches, such as ACT, KOH, TMAH, HF (liquid);oxygen plasma, SCCO₂, or super critical CO₂ (the use of super criticalCO₂ as an etchant is described in U.S. patent application Ser. No.10/167,272, which is incorporated herein by reference). Of course, theetchants and methods selected should be matched to the sacrificialmaterials being removed and the desired materials being left behind.

[0086] It will be appreciated by those of skill in the art that a newand useful spatial light modulator has been described herein. In view ofthe many possible embodiments to which the principles of this inventionmay be applied, however, it should be recognized that the embodimentsdescribed herein with respect to the drawing figures are meant to beillustrative only and should not be taken as limiting the scope ofinvention. For example, those of skill in the art will recognize thatthe illustrated embodiments can be modified in arrangement and detailwithout departing from the spirit of the invention. In particular, eachof the layers of the structure layers, such as micromirror plate layer300 (which may further comprises layers 301, 303, 305 and 307 as shownin FIG. 6B), hinge support layers 340 and 350, and hinge layer 360 maycomprise one or more of a number of suitable materials that are eitherelectro-conducting or electro-insulating, as long as at least one of thelayers is electro-conducting and provides electro-contact to themicromirror. In another example, the Sandia SUMMiT process (usingpolysilicon for structural layers) or the Cronos MUMPS process (alsopolysilicon for structural layers) could be used in the presentinvention. Also, a MOSIS process (AMI ABN−1.5 um CMOS process) could beadapted for the present invention, as could a MUSiC process (usingpolycrystalline SiC for the structural layers) as disclosed, forexample, in Mehregany et al., Thin Solid Films, v. 355-356, pp. 5¹⁸-524,1999. Also, the sacrificial layer and etchant disclosed herein areexemplary only. For example, a silicon dioxide sacrificial layer couldbe used and removed with HF (or HF/HCl), or a silicon sacrificial couldbe removed with CIF₃ or BrF₃. Also a PSG sacrificial layer could beremoved with buffered HF, or an organic sacrificial such as polyimidecould be removed in a dry plasma oxygen release step. Of course theetchant and sacrificial material should be selected depending upon thestructural material to be used. Also, though PVD and CVD are referred toabove, other thin film deposition methods could be used for depositingthe layers, including spin-on, sputtering, anodization, oxidation,electroplating and evaporation. Therefore, the invention as describedherein contemplates all such embodiments as may come within the scope ofthe following claims and equivalents thereof.

We claim:
 1. A spatial light modulator comprising an array ofmicromirrors, each micromirror comprising: a hinge; and a micromirrorplate held via the hinge on a substrate, the micromirror plate beingdisposed in a plane separate from the hinge and having a diagonalextending across the micromirror plate, the micromirror plate beingattached to the hinge such that the micromirror plate can rotate along arotation axis that is parallel to, but off-set from the diagonal of themicromirror plate.
 2. The spatial light modulator of claim 1, whereinthe hinge is disposed on an opposite side of the micromirror plate fromthe substrate.
 3. The spatial light modulator of claim 2, wherein thesubstrate is a substrate transmissive to visible light.
 4. The spatiallight modulator of claim 3, wherein a second substrate having electrodesand circuitry thereon is positioned proximate to the substratetransmissive to visible light for electrostatically deflecting themicromirrors.
 5. The spatial light modulator of claim 2, wherein themicromirror plate is substantially square.
 6. The spatial lightmodulator of claim 2, wherein the micromirror plate has a substantiallyrhombus shape.
 7. The spatial light modulator of claim 2, wherein themicromirror plate has a substantially trapezoidal shape.
 8. The spatiallight modulator of claim 2, wherein the micromirror plate has asubstantially rectangular shape.
 9. The spatial light modulator of claim1, wherein the hinge is a torsion hinge and the mirror plate rotatesalong an axis of rotation corresponding to the location of the torsionhinge.
 10. The spatial light modulator of claim 1, wherein axis ofrotation is located 0.5 micrometers or more away from the micromirrorplate diagonal.
 11. The spatial light modulator of claim 10, wherein theaxis of rotation is located 1.0 micrometers or more away from themicromirror plate diagonal.
 12. The spatial light modulator of claim 11,wherein the axis of rotation is located 2.0 micrometers or more awayfrom the micromirror plate diagonal.
 13. The spatial light modulator ofclaim 1, wherein an edge of the micromirror plate is covered by a lightabsorbing material.
 14. The spatial light modulator of claim 1, that ispart of a projection system for forming an image on a target.
 15. Thespatial light modulator of claim 2, wherein the substrate is a glass orquartz substrate.
 16. The spatial light modulator of claim 15, wherein asurface of the substrate is covered with an anti-reflection film. 17.The spatial light modulator of claim 1, wherein the micromirror plate isheld on the substrate by one or more posts that connect the hinge to thesubstrate.
 18. The spatial light modulator of claim 17, wherein thepoint of attachment of the hinge to the micromirror plate is not alongthe diagonal of the micromirror plate.
 19. The spatial light modulatorof claim 17, wherein the wherein the micromirror plate is held on thesubstrate by two posts.
 20. The spatial light modulator of claim 19,wherein the point of attachment of the hinge to the micromirror plate isnot between the two posts.
 21. The spatial light modulator of claim 19,wherein the two posts are positioned at two ends of the diagonal of themicromirror plate.
 22. The spatial light modulator of claim 14, whereinthe substrate comprises one or more electrodes and circuitry forelectrically controlling the motion of the micromirror plate.
 23. Thespatial light modulator of claim 18, wherein the point of attachment ofthe hinge to the micromirror plate is at least 0.5 micrometers away fromthe diagonal of the micromirror plate.
 24. The spatial light modulatorof claim 23, wherein the point of attachment is at least 1.0 micrometersaway from the diagonal.
 25. The spatial light modulator of claim 1,wherein the hinge is disposed between the micromirror plate and thesubstrate.
 26. The spatial light modulator of claim 1, wherein thesubstrate is a semiconductor substrate.
 27. The spatial light modulatorof claim 1, wherein the ON state of a micromirror is defined as a pointat which the rotation of the micromirror is stopped such that light fromthe micromirror is capable of being viewed on a target as a pixel in aprojected image.
 28. The spatial light modulator of claim 27, whereinthe micromirror abuts against the substrate or structure patterned onthe substrate so that further rotation of the micromirror plate isprevented in the ON state.
 29. The spatial light modulator of claim 27,wherein each micromirror comprises a support post for connecting thehinge to the substrate and a stopping mechanism projecting held by thesupport post for resisting rotation of the micromirror plate.
 30. Thespatial light modulator of claim 29, wherein when the micromirror plateabuts against the stopping mechanism when it reaches the ON state. 31.The spatial light modulator of claim 27, wherein the OFF state of themicromirrors is at an angle of at least −2 degrees relative to thesubstrate.
 32. The spatial light modulator of claim 31, wherein the OFFstate of the micromirrors is at an angle of at least −3 degrees relativeto the substrate.
 33. The spatial light modulator of claim 32, wherein aset of electrodes are provided for rotating the micromirror to the ONstate, and a second set of electrodes are provided for rotating themicromirror to the OFF state.
 34. The spatial light modulator of claim1, wherein the hinge is disposed in a plane separate from the plane ofthe micromirror plate by a gap of at least 0.1 micrometers.
 35. Thespatial light modulator of claim 31, wherein the hinge is part of ahinge support structure that rotatably holds the micromirror plate onthe substrate, wherein the hinge support structure is a multilayerstructure that has a curved state, due to stress differences between thelayers, when the micromirror plate is not electrostatically deflected.36. The spatial light modulator of claim 1, wherein the micromirrorplate has a diagonal length of 25 microns or less.
 37. The spatial lightmodulator of claim 1, wherein the point of attachment of the hinge tothe micromirror plate is located at a point away from the diagonal ofthe micromirror plate at a distance from {fraction (1/40)} to ⅓ thelength of the diagonal.
 38. The spatial light modulator of claim 37,wherein the point of attachment of the hinge to the micromirror plate islocated at a point away from the diagonal of the micromirror plate at adistance from {fraction (1/20)} to ¼ the length of the diagonal.
 39. Thespatial light modulator of claim 1, wherein each micromirror comprises asupport post for connecting the hinge to the substrate and a stoppingmechanism projecting held by the support post for resisting rotation ofthe micromirror plate, wherein the micromirror impacts the stoppingmechanism in the ON state, wherein the micromirror plate is asubstantially four sided plate having a diagonal, and wherein the hingeattaches to the micromirror plate at a point at least 0.5 um away fromthe diagonal, and wherein the hinge is disposed in a plane separate fromthe mirror plate by a gap of at least 0.1 um.
 40. The spatial lightmodulator of claim 1, wherein the hinge comprises a nitride of titaniumand/or silicon.
 41. A spatial light modulator comprising an array ofmicromirrors on a substrate, each micromirror comprising: two posts; ahinge; and a micromirror plate held on the substrate via the hinge andtwo posts, the micromirror plate being disposed in a plane separate fromthe hinge and having a diagonal extending across the micromirror plate,the micromirror plate being attached to the hinge at a point not along astraight line between the two posts.
 42. The spatial light modulator ofclaim 41, wherein the hinge is disposed on an opposite side of themicromirror plate from the substrate.
 43. The spatial light modulator ofclaim 42, wherein the substrate is a substrate transmissive to visiblelight.
 44. The spatial light modulator of claim 43, wherein a secondsubstrate having electrodes and circuitry thereon is positionedproximate to the substrate transmissive to visible light forelectrostatically deflecting the micromirrors.
 45. The spatial lightmodulator of claim 42, wherein the micromirror plate is substantiallysquare.
 46. The spatial light modulator of claim 42, wherein themicromirror plate has a substantially rhombus shape.
 47. The spatiallight modulator of claim 42, wherein the micromirror plate has asubstantially trapezoidal shape.
 48. The spatial light modulator ofclaim 42, wherein the micromirror plate has a substantially rectangularshape.
 49. The spatial light modulator of claim 41, wherein the hinge isa torsion hinge and the mirror plate rotates along an axis of rotationcorresponding to the location of the torsion hinge.
 50. The spatiallight modulator of claim 41, wherein axis of rotation is located 0.5micrometers or more away from the micromirror plate diagonal.
 51. Thespatial light modulator of claim 50, wherein the axis of rotation islocated 1.0 micrometers or more away from the micromirror platediagonal.
 52. The spatial light modulator of claim 51, wherein the axisof rotation is located 2.0 micrometers or more away from the micromirrorplate diagonal.
 53. The spatial light modulator of claim 41, wherein anedge of the micromirror plate is covered by a light absorbing material.54. The spatial light modulator of claim 41, that is part of aprojection system for forming an image on a target.
 55. The spatiallight modulator of claim 42, wherein the substrate is a glass or quartzsubstrate.
 56. The spatial light modulator of claim 55, wherein asurface of the substrate is covered with an anti-reflection film. 57.The spatial light modulator of claim 41, wherein the micromirror plateis held on the substrate by one or more posts that connect the hinge tothe substrate.
 58. The spatial light modulator of claim 57, wherein thepoint of attachment of the hinge to the micromirror plate is not alongthe diagonal of the micromirror plate.
 59. The spatial light modulatorof claim 57, wherein the wherein the micromirror plate is held on thesubstrate by two posts.
 60. The spatial light modulator of claim 59,wherein the point of attachment of the hinge to the micromirror plate isnot between the two posts.
 61. The spatial light modulator of claim 59,wherein the two posts are positioned at two ends of the diagonal of themicromirror plate.
 62. The spatial light modulator of claim 54, whereinthe substrate comprises one or more electrodes and circuitry forelectrically controlling the motion of the micromirror plate.
 63. Thespatial light modulator of claim 58, wherein the point of attachment ofthe hinge to the micromirror plate is at least 0.5 micrometers away fromthe diagonal of the micromirror plate.
 64. The spatial light modulatorof claim 63, wherein the point of attachment is at least 1.0 micrometersaway from the diagonal.
 65. The spatial light modulator of claim 41,wherein the hinge is disposed between the micromirror plate and thesubstrate.
 66. The spatial light modulator of claim 41, wherein thesubstrate is a semiconductor substrate.
 67. The spatial light modulatorof claim 41, wherein the ON state of a micromirror is defined as a pointat which the rotation of the micromirror is stopped such that light fromthe micromirror is capable of being viewed on a target as a pixel in aprojected image.
 68. The spatial light modulator of claim 67, whereinthe micromirror abuts against the substrate or structure patterned onthe substrate so that further rotation of the micromirror plate isprevented in the ON state.
 69. The spatial light modulator of claim 67,wherein each micromirror comprises a support post for connecting thehinge to the substrate and a stopping mechanism projecting held by thesupport post for resisting rotation of the micromirror plate.
 70. Thespatial light modulator of claim 69, wherein when the micromirror plateabuts against the stopping mechanism when it reaches the ON state. 71.The spatial light modulator of claim 67, wherein the OFF state of themicromirrors is at an angle of at least −2 degrees relative to thesubstrate.
 72. The spatial light modulator of claim 71, wherein the OFFstate of the micromirrors is at an angle of at least −3 degrees relativeto the substrate.
 73. The spatial light modulator of claim 72, wherein aset of electrodes are provided for rotating the micromirror to the ONstate, and a second set of electrodes are provided for rotating themicromirror to the OFF state.
 74. The spatial light modulator of claim41, wherein the hinge is disposed in a plane separate from themicromirror plate by a gap of at least 0.1 micrometers.
 75. The spatiallight modulator of claim 71, wherein the hinge is part of a hingesupport structure that rotatably holds the micromirror plate on thesubstrate, wherein the hinge support structure is a multilayer structurethat has a curved state, due to stress differences between the layers,when the micromirror plate is not electrostatically deflected.
 76. Thespatial light modulator of claim 41, wherein the micromirror plate has adiagonal length of 25 microns or less.
 77. The spatial light modulatorof claim 41, wherein the point of attachment of the hinge to themicromirror plate is located at a point away from the diagonal of themicromirror plate at a distance from {fraction (1/40)} to ⅓ the lengthof the diagonal.
 78. The spatial light modulator of claim 67, whereinthe point of attachment of the hinge to the micromirror plate is locatedat a point away from the diagonal of the micromirror plate at a distancefrom {fraction (1/20)} to ¼ the length of the diagonal.
 79. The spatiallight modulator of claim 41, wherein each micromirror comprises asupport post for connecting the hinge to the substrate and a stoppingmechanism projecting held by the support post for resisting rotation ofthe micromirror plate, wherein the micromirror impacts the stoppingmechanism in the ON state, wherein the micromirror plate is asubstantially four sided plate having a diagonal, and wherein the hingeattaches to the micromirror plate at a point at least 0.5 um away fromthe diagonal, and wherein the hinge is disposed in a plane separate fromthe mirror plate by a gap of at least 0.1 um.
 80. The spatial lightmodulator of claim 41, wherein the hinge comprises a nitride of titaniumand/or silicon.
 81. A projection system comprising: a light source; thespatial light modulator of claim 1; condensing optics, wherein lightfrom the light source is focused onto the array of micro-mirrors;projection optics for projecting light selectively reflected from thearray of micro-mirrors; and a controller for selectively actuatingmicro-mirrors in the array of micro-mirrors.
 82. A projection systemcomprising: a light source; the spatial light modulator of claim 41;condensing optics, wherein light from the light source is focused ontothe array of micro-mirrors; projection optics for projecting lightselectively reflected from the array of micro-mirrors; and a controllerfor selectively actuating micro-mirrors in the array of micro-mirrors.83. A method comprising: providing a substrate; depositing a firstsacrificial layer; forming a micromirror plate having a diagonal;depositing a second sacrificial layer; patterning the sacrificial layersto form two vias down to the substrate and a via down to the micromirrorplate, the three vias forming a triangle; forming a hinge-structurecomprising posts in the three vias and a hinge structure therebetweenfor connecting the micromirror plate to the substrate; and removing thesacrificial layers.
 84. The method of claim 83, wherein the sacrificiallayers comprise amorphous silicon.
 85. The method of claim 84, whereinthe sacrificial layers are removed with a spontaneous gas phase chemicaletchant.
 86. The method of claim 85, wherein the etchant is a noble gasfluoride or an interhalogen.
 87. The method of claim 83, wherein thesacrificial layers comprise a polymer.
 88. A method comprising:providing a substrate; depositing a first sacrificial layer; forming amicromirror plate having a diagonal; depositing a second sacrificiallayer; forming a hinge-structure on the substrate that attaches to themicromirror plate at a point not along the diagonal, the hinge structurefor holding the micromirror plate above the substrate such that themicromirror plate can rotate by means of a rotation axis that isparallel to, but off-set from the diagonal of the micromirror plate; andremoving the first and second sacrificial layers.
 89. The method ofclaim 88, wherein the sacrificial layers comprise amorphous silicon. 90.The method of claim 89, wherein the sacrificial layers are removed witha spontaneous gas phase chemical etchant.
 91. The method of claim 90,wherein the etchant is a noble gas fluoride or an interhalogen.
 92. Themethod of claim 88, wherein the sacrificial layers comprise a polymer.93. A reflective micromirror device, comprising: a substrate; amicromirror plate held on the substrate for reflecting incident light;and a hinge-structure connecting the micromirror plate to the substrate,wherein the hinge structure is a multilayer structure having layers ofdifferent intrinsic stress such that the hinge structure is curved dueto the difference in intrinsic stress.
 94. The reflective micromirrordevice of claim 93, wherein one layer of the multilayer structure iscompressive and another layer of the multilayer structure is tensile.95. A method for forming a micromirror for a spatial light modulator,comprising: providing a substrate; depositing a first sacrificial layer;forming a micromirror plate on the first sacrificial layer forreflecting an incident light; depositing a second sacrificial layer onthe micromirror plate; forming a hinge-structure on the secondsacrificial layer for connecting the micromirror plate to the substrate,wherein the forming of the hinge structure comprises depositing a firsthinge-structure layer having an intrinsic positive tensile-strain, anddepositing a second hinge-structure layer on the first layer, whereinthe second layer has an intrinsic negative compression strain; andremoving the first and second sacrificial layers such that the first andsecond hinge-structure layers are curved at their natural resting statesdue to the difference in intrinsic strain, such that the micromirrorplate is held by the hinge-structure in a position not parallel to thesubstrate.
 96. The method of claim 96, wherein the first hinge structurelayer is deposited by chemical vapor deposition.
 97. The method of claim96, wherein the second hinge structure layer is deposited by physicalvapor deposition.
 98. The method of claim 96, wherein the second hingestructure layer is a reactively sputtered ceramic layer and the firsthinge structure layer is a ceramic layer deposited by chemical vapordeposition.